1. Field of the Invention
The present invention relates to an image formation apparatus such as a color laser beam printer and a multifunctional device having a plurality of laser scanning units. More specifically, the present invention relates to a ring oscillator setting apparatus and method depending on environmental changes of the image formation apparatus to set a ring oscillator having an optimal loopspeed to generate a video clock.
2. Description of Related Art
In conventional image formation apparatuses, a laser printer modulator includes a ring oscillator, a loopspeed detection unit having a modulator, a clock generator with a ring oscillator, a loopspeed detection unit, and a clock generation unit.
The ring oscillator generates a predetermined frequency. The ring oscillator includes an odd number of inverter chains. In general, an inverter block is constructed by connecting three or five inverters in series, and serves as one tab. The ring oscillator is sensitive to noise so that its oscillation frequency may vary depending on change of a design, temperature, and voltage. A loopspeed refers to the number of pulses generated at the oscillation frequency by the ring oscillator for a predetermined unit time.
A circuit for video clock generation and modulation uses the ring oscillator. The circuit for video clock generation is closely related to the ring oscillator and the loopspeed for changing and modulating frequency. The oscillation frequency of the ring oscillator may vary depending on changes in the external environment such as temperature and voltage. Consequently, changes in the external environment may lead to image quality degradation. Accordingly, in order to minimize change of the oscillation frequency of the ring oscillator, a method utilizing the step of driving static current in the power supply unit of the ring oscillator can be employed. However, with the above-identified method, it is relatively difficult to cope with changes in external temperature. Moreover, when a variety of video clocks are used, it is necessary to have a large internal operational circuit of a video clock generator and a modulator to deal with all of wideband frequency ranges. Therefore, problems with image quality degradation may arise including damage of video data. Damage of video data is caused by the modulator changing the loopspeed of the ring oscillator due to changes in the system environment such as temperature change.
Accordingly, there is a need for an image formation apparatus having an improved ring oscillator setting apparatus that has a relatively optimal loopspeed even when the external environment changes.